Recently, the need for very wideband RF power amplifiers has significantly increased due to new wireless communication technologies, particular in transceivers used in mobile application. Therefore, there is a need for a single multi-mode multi-band power amplifier module, which can support communication technologies such as Wideband Code Division Multiple Access (WCDMA), Global System for Mobile (GSM), Communications, Enhanced Data Rates for GSM Evolution (EDGE), and Long Term Evolution (LTE) technologies. However, obtaining a high-performance wideband RF power amplifier for multi-mode multi-band radio is difficult, particularly the harmonic suppression.
The harmonic suppression is one important performance criterion in ensuring multi-user communication by limiting the emission of nonlinear harmonics that arise from the nonlinearity of RF power amplifiers, mainly due to the distortion.
Second-order harmonics are the most important to suppress for wideband RF power amplifiers because of the closest frequency space to the operation frequency band, and the strong power among all of the harmonic tones. For example, when the bandwidth of a power amplifier covers from 1.5 GHz to 2.7 GHz, the lowest second-order harmonic is at 3 GHz, introducing a difficult design issue on the output matching network (OMN). The highest in-band channel at 2.7 GHz should have a low insertion loss, while the lowest second-order harmonic at 3 GHz should have a high suppression from the output matching network. This demands output matching network design for providing sufficient second-order harmonic suppression, normally better than 30 decibels relative to the carrier (dBc), while not affecting the in-band operation.
FIG. 2 shows a prior art of a multi-band power amplifier that fulfills the necessary second harmonic suppression requirement through multi-chain topology. Two power amplifiers 201 and 203 are designed independently to provide second order harmonic suppression in narrow band A and band B. Compared to a single wide band power amplifier, this multi-chain architecture requires a switch 205 connected with the output matching networks 202 and 204. The output matching network (OMN) 202 and 204 are designed such that the second order harmonic suppression at each narrow band A and B is achieved. The design requirements of OMN 202 and 204 for multi-chain architecture are less challenging than a single OMN for a wideband power amplifier, but the overall circuit is more complex and large profile.
FIG. 3 shows another prior art multiband power amplifier, which achieves the second harmonic suppression using a reconfigurable output matching networks. A multi band power amplifier 302 with an input matching network (IMN) 301 and an output matching network (OMN) 303 provides impedance matching and harmonic rejection for band A, when switch 304 is OFF. When the power amplifier 302 operates for band B, OMN 305 is combined with the 303 with switch 304 ON, so that that impedance matching and second harmonic suppression for band B is achieved. The disadvantage of this reconfigurable output matching network is switch 304. Although the switch enables a reconfigurable output matching optimized for each single band, the insertion loss from the switch can significantly reduce the power efficiency of the overall multi-mode power amplifier with additional cost and complexity.
FIG. 4 shows another prior art multiband power amplifier, which achieves the second harmonic suppression with multiple band suppression filters in the output matching network of a power amplifier. A filter 401 includes one wideband low-pass filter and four band-suppression filters (410, 420, 430 and 440). Each band-suppression filter is a series LC tank. The bandwidth of each band-suppression filter at stop band is determined by the Q of the tank. For a low insertion loss, the tank Q should be higher, which in turn reduces the stop-band bandwidth.
The required suppression band for second harmonic is 2×(fH−fL), which is two times wider than the pass-band bandwidth (fH−fL). Hence, for wideband multi-mode operation, a single band-elimination filter cannot provide the stop-band bandwidth over 2×(fH−fL). Instead, several band-elimination filters resonating at different frequencies are necessary to provide a wide stop-band bandwidth, which results in a large insertion loss from the additional passive elements in the output matching network and large profile.
In a summary, the power efficiency degradation and the increased cost and area requirement for the multiple band-elimination filters are the primary disadvantages of prior art wideband multi-mode power amplifiers.